But Mary knew she had been saved from sin. The angel greeted her with this unique greeting. David never had that!
Huh? Mary’s greeting was hardly unique. Zacharias was first greeted and told of how special his son would be.
Manoah’s wife, the mother of Samson, was greeted in much the same way as Mary, almost identically (Judges 13).
Judges 13:3 And the angel of the LORD appeared unto the woman, and said unto her, Behold now, thou art barren, and bearest not: but thou shalt conceive, and bear a son.
4 Now therefore beware, I pray thee, and drink not wine nor strong drink, and eat not any unclean thing:
5 For, lo, thou shalt conceive, and bear a son; and no razor shall come on his head: for the child shall be a Nazarite unto God from the womb: and he shall begin to deliver Israel out of the hand of the Philistines.
6 Then the woman came and told her husband, saying, A man of God came unto me, and his countenance was like the countenance of an angel of God, very terrible: but I asked him not whence he was, neither told he me his name:
7 But he said unto me, Behold, thou shalt conceive, and bear a son; and now drink no wine nor strong drink, neither eat any unclean thing: for the child shall be a Nazarite to God from the womb to the day of his death.
Hannah, too, was blessed and her prayer of thanks very similar to Mary’s. (1 Samuel 2)
Gideon in Judges 6:12 was greeted with “The Lord is with thee, thou mighty man of valour”.
Daniel in Daniel 9:23 was greeted with “O Daniel, I am come forth to give thee skill and understanding… for thou art greatly beloved.” (also echoed in Daniel 10 verses 11 and 19)
Mary’s hymn of praise was not a new thing, and apparently common to those who had been barren but granted childbirth by God’s power:
Isaiah 54:1 Sing, O barren, thou that didst not bear; break forth into singing, and cry aloud, thou that didst not travail with child: for more are the children of the desolate than the children of the married wife, saith the LORD.